Materials which closely mimic the ultrasonic propagation characteristics of human tissue are employed in imaging phantoms and other test objects for use with ultrasound scanners. These phantoms may be used to carry out performance checks on ultrasound scanners. Phantoms may also be used for training or testing student technologists in the operation of ultrasound scanners or the interpretation of ultrasound images produced by such scanners.
A phantom containing tissue mimicking material is disclosed in U.S. Pat. No. 4,277,367, to Madsen, et al., entitled Phantom Material and Methods, in which both the speed of sound and the ultrasonic attenuation properties could be simultaneously controlled in a mimicking material based on water based gels, such as those derived from animal hides. In one embodiment, ultrasound phantoms embodying the desired features for mimicking soft tissue were prepared from a mixture of gelatin, water, n-propanol and graphite powder, with a preservative. In another embodiment, an oil and gelatin mixture formed the basis of the tissue mimicking material.
Tissue mimicking material is typically used to form the body of an ultrasound scanner phantom. This is accomplished by enclosing the material in a container which is closed by an ultrasound transmitting window cover. The tissue mimicking material is admitted to the container in such a way as to exclude air bubbles from forming in the container. In addition to the tissue mimicking material itself, scattering particles, spaced sufficiently close to each other that an ultrasound scanner is incapable of resolving individual scattering particles, and testing spheres or other targets, may be located within the phantom container, suspended in the tissue mimicking material body. Such an ultrasound phantom is useful in evaluating the ability of ultrasound medical diagnostic scanners to resolve target objects of selected sizes located throughout the tissue mimicking material. The objective is for the ultrasound scanner to resolve the testing spheres or other targets from the background material and scattering particles. This type of ultrasound phantom is described in U.S. Pat. No. 4,843,866, to Madsen, et al., entitled Ultrasound Phantom.
U.S. Pat. No. 5,625,137 to Madsen, et al. discloses a tissue mimicking material for phantoms with very low acoustic backscatter coefficient that may be in liquid or solid form. A component in both the liquid and solid forms is a filtered aqueous mixture of large organic water soluble molecules and an emulsion of fatty acid esters, which may be based on a combination of condensed milk and water. Hydroxy compounds, such as n-propanol, can be used to control the ultrasonic speed of propagation through the material and a preservative from bacterial invasion can also be included. The use of scattering particles allows a very broad range of relative backscatter levels to be achieved.
The tissue mimicking material in ultrasound phantoms is typically water based to best simulate human tissue. The tissue mimicking material may be either in liquid form or in a solid gel form. As discussed in greater detail in the foregoing patents, the tissue mimicking material is typically held within a container having a bottom and upright walls which may be molded as a unit or formed of flat pieces of plastic or other materials glued or otherwise joined together. Such rigid walls would not permit the transmission of ultrasound from a transducer through the walls to the tissue mimicking material within the container. The container cannot be left open to expose the tissue mimicking material since to do so would allow evaporation of the water within the material as well as potential oxidation or contamination of the material. Thus, the open tops of the phantom containers are typically sealed with a thin sheet of plastic such as polyurethane or saran (e.g., Saran Wrap.RTM.) to protect the tissue mimicking material from physical contact with the transducer and to minimize moisture loss from the material. The thin sheets of plastic are flexible and thin enough to transmit ultrasound therethrough to the tissue mimicking material without substantial attenuation or degradation of the signal, but are not entirely impervious to water vapor or air gas molecules such as oxygen. Consequently, gel based phantoms have commonly been made using a layer of oil based gel covering the tissue mimicking material gel, with the flexible plastic cover in contact with the oil based gel. The oil based gel reduces water loss through the window but does not entirely eliminate it, and the use of an oil gel adds an additional layer of material through which the ultrasound must be transmitted from the transducer to the tissue mimicking material and back again. The oil based gels can detach from the plastic cover and walls of the phantom, allowing rapid desiccation of the phantom contents. In addition, even with the oil based gel in proper position, significant desiccation can occur.